Enthalpy_of_Neutralization

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Enthalpy of Neutralization

Introduction

Energy changes always accompany chemical reactions. If energy, in the form of heat, isliberated the reaction is exothermic and if energy is absorbed the reaction is endothermic.

Thermochemistry is concerned with the measurement of the amount of heat evolved or

absorbed. The heat (or enthalpy) of neutralization (H) is the heat evolved when anacid and a base react to form a salt plus water.

Eq. 1 HNO2(aq) + NAOH(aq) NaNO2(aq) + H2O(l) + Q

Q in the above equation is -H and is expressed in kJ/mol of water. Neutralization

reactions are generally exothermic and thus H is negative.

Heat measurements are performed by carrying out the reaction in a special containercalled a calorimeter. The heat (Q) given off by the neutralization reaction is absorbed by

the reaction solution and the calorimeter. Both the solution and calorimeter increase intemperature due to the absorbed heat and this increase can be measured with a

thermometer. H is negative if heat is evolved and positive if heat is absorbed.

Eq. 2 -Hneutralization = QSolution + QCalorimeter

To evaluate the calorimeter constant (also known as its heat capacity) in J/oC, one adds a

known mass of hot water to a known mass of cold water which is in the calorimeter. Heat

(Q) is lost by the hot water and is absorbed by the cold waterand the calorimeter. Thusthe heat absorbed by the calorimeter is the heat lost by the hot water minus the heat

gained by the cold water.

QHot water = QCold water + QCalorimeter

Eq. 3 Qcalorimeter = QHot water - QCold water

It should be noted that we assume that the temperature of the calorimeter is the same as

the solution inside it at all times. Q for both the hot and cold water is given by:

Eq. 4 Q = (4.184J/g-oC)(Mass in g)(t)

t is found by plotting temperature versus time for the system in the calorimeter andextrapolating the results to find t at the instant of mixing (in this experiment, 5 minutes).

A typical graph is shown in Figure 1.

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Hot Water

t hot waterTemperature,

oC

Mixture

t cold waterand calorimeter

Cold Water5.0

Time in Minutes

Figure 1

The heat gained by the calorimeter is the difference between the heat lost by the hot waterand the heat gained by the cold water. The calorimeter constant is this difference divided

by the temperature change of the calorimeter (temperature change of the cold water)

Eq. 5 Calorimeter constant = QCalorimeter/tCold water

The H of neutralization is found by mixing known quantities (moles) of an acid and a

base (both initially at the same temperature) in a calorimeter and measuring t of the

mixture and the calorimeter. A typical graph for neutralization is shown in Figure 2.

Salt Solution

Temperature,oC

t of salt solution and calorimeter

Acid and Base

5.0

Time in Minutes

Figure 2

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The heat given off by the neutralization reaction, H, is the sum of the heat absorbed by

the solution and calorimeter.

Eq. 6 -H = + Qsolution + Qcalorimeter

Eq. 7 Qsolution = (Sp. Ht.)(Volume)(Density)(

t)

Eq. 8 Qcalorimeter = (Calorimeter Constant)(t)

The specific heat (Sp. Ht.) and the density of the solution of the salt formed from your

assigned acid and base. You must calculate the H forone mole of water formed in your

system in order to compare it to the theoretical value. t is the same for the solution andthe calorimeter.

Procedure

Calorimeter Constant

Weigh 50.0 grams of distilled water into your assigned calorimeter. Be sure it is clean

and dry. This is the cold water. Suspend the supplied 50oC thermometer into the water

from a paper clip on a ring stand. Be sure to note the thermometercan be read to

0.01oC.

Weigh 50.0 grams of distilled water into a 100 mL beaker. Heat this water to 15 20oC

above the temperature of the cold water. Remove from the hot plate and place on the

bench. Suspend a 110oC thermometer (in your locker) from a ring stand with a paper clip.

Be sure to note the starting time when measuring temperatures. Starting with the hot

water, alternately measure the hot and cold water temperatures for a 4.5 minute period.You will have 5 measurements of each water sample. At the 5.0 minute mark quickly add

the hot water to the cold water (in the calorimeter) and record the temperature of the

mixture at 1 minute intervals for a 10 minutes for a total of 15 readings.

Clean and dry the beaker and calorimeter and repeat the above procedure for a second

trial. Clean and dry the beaker and calorimeter for the neutralization reaction.

Plot graphs of temperature vs time for the two calorimeter constant trials. Extrapolate the

three lines beyond the data points and evaluate the hot and cold water temperature

changes (t) at the 5 minute mark as shown in Figure 1. Evaluate the averagecalorimeter constant on page 9.

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Procedure

Acid-Base Neutralization

Each student will be assigned an acid and a base from the following list.

Acids: 2.00 M HCl Bases: 2.00 M NaOH

2.00 M HNO3 2.00 M KOH1.00 M H2SO4 2.00 M NH4OH

0.667 M H3PO4

Pipet 50.00 mL of your assigned acid into the clean dry calorimeter. Suspend the

50oC thermometer in the acid. Pipet 50.00 mL of your assigned base into a 100

mL beaker. Since both the acid and base have been at room temperature forseveral hours, we can safely assume that both are at the same temperature.

Record the acid (and base) temperature every one half minutes for 4.5 minutes.At the 5 minute mark quickly add the base to the acid in the calorimeter. Record

the solution temperature every minute for 10 minutes giving a total of 15 minutesof readings. Record all temperatures to the nearest 0.01

oC.

Clean and dry the beaker and calorimeter and repeat the above procedure for asecond trial.

Plot graphs of temperature vs time for the two neutralization reaction trials.Extrapolate the two lines beyond the data points and determine the temperature

changes (t) at the 5 minute mark as shown in Figure 2. Evaluate the averageH for the reaction on page 10.

Write a balanced equation for you assigned acid-base mixture, be sure toneutralize all of the acidic hydrogens of your acid. Determine the moles of water

formed in your system from the molarity and volumes of acid and base used and

correct yourH for the formation ofone mole of water.

Using standard thermodynamic tables calculate the theoretical enthalpy change in

kJ/1 mole water and compare to you experimentally determined value.

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Name:_________________________________

Data

Calorimeter Constant, Trial 1

Time, Minutes Hot Water Temp,oC Cold Water Temp,

oC Mixture Temp,

oC

0.0 ---------- -----------

0.5 ---------- ----------

1.0 ---------- ----------

1.5 ---------- ----------

2.0 ---------- ----------

2.5 ---------- ----------

3.0 ---------- ----------

3.5 ---------- ----------

4.0 ---------- ----------

4.5 ---------- ----------

5.0 Mix The Samples

6.0 ---------- ----------

7.0 ---------- ----------

8.0 ---------- ----------

9.0 ---------- ----------

10.0 ---------- ----------

11.0 ---------- ----------

12.0 ---------- ----------

13.0 ---------- ----------

14.0 ---------- ----------

15.0 ---------- ----------

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Name:_________________________________

Data

Calorimeter Constant, Trial 2

Time, Minutes Hot Water Temp,oC Cold Water Temp,

oC Mixture Temp,

oC

0.0 ---------- ----------

0.5 ---------- ----------

1.0

---------- ----------

1.5

---------- ----------

2.0

---------- ----------

2.5

---------- ----------

3.0---------- ----------

3.5

--------- ----------

4.0

---------- ----------

4.5

---------- ----------

5.0

Mix The Samples

6.0

---------- ----------

7.0

---------- ----------

8.0

---------- ----------

9.0

---------- ----------

10.0

---------- ----------

11.0

---------- ----------

12.0---------- ----------

13.0

---------- ----------

14.0

---------- ----------

15.0

---------- ----------

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Name:_________________________________

Data

Acid-Base Neutralization, Trial 1

Time, Minutes Acid and Base Temperatures,oC Salt Solution Temperature,

oC

0.0 ----------

0.5 ----------

1.0

----------

1.5

----------

2.0

----------

2.5

----------

3.0----------

3.5

----------

4.0

----------

4.5

----------

5.0

Mix the Acid and Base

6.0

----------

7.0

----------

8.0

----------

9.0

----------

10.0

----------

11.0

----------

12.0----------

13.0

----------

14.0

----------

15.0

----------

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Name:_________________________________

Data

Acid-Base Neutralization, Trial 2

Time, Minutes Acid and Base Temperatures,oC Salt Solution Temperature,

oC

0.0

----------

0.5

----------

1.0

----------

1.5

----------

2.0

----------

2.5

----------

3.0---------

3.5

----------

4.0

----------

4.5

---------

5.0

Mix the Acid and Base

6.0

---------

7.0

----------

8.0

----------

9.0

----------

10.0

----------

11.0

----------

12.0---------

13.0

----------

14.0

----------

15.0

----------

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Name:_________________________________

Calculations and Results

Calorimeter Constant Determination

Trial 1.

1. Mass of hot water (g) ________________________________2. t of hot water (

oC) (absolute value) _____________________

3. Heat lost by hot water (J) _____________________________

4. Mass of cold water (g) _______________________________5. t of cold water (oC) ________________________________6. Heat gained by cold water (J) _________________________7. Heat gained by calorimeter (J) (3 6) ___________________8. Calorimeter Constant (J/oC) (7/5) _______________________

Trial 2

1. Mass of hot water (g) _______________________________2. t of hot water (oC)(absolute value) ____________________3. Heat lost by hot water (J) ____________________________4. Mass of cold water (g) _______________________________5. t of cold water (oC) ________________________________6. Heat gained by cold water (J) _________________________7. Heat gained by calorimeter (J) (3 6) ___________________8. Calorimeter Constant (J/oC) (7/5) _______________________

Average Calorimeter Constant Value (J/oC) _______________

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Name:_________________________________



Assigned Acid and Base: ______________ _________________

Balanced Equation for Complete Neutralization

Density of Salt Solution (g/mL) ____________________________

Specific Heat of Salt Solution (J/g-oC) _______________________

Trial 1

1. Volume of Salt Solution (mL) ________________________________2. Mass of Salt Solution (g) ____________________________________3. t of Salt Solution (oC) (from graph) __________________________4. Heat Gained by Salt Solution (J) ______________________________5. Heat Gained by Calorimeter (J) (Cal. Const. x 3) _________________6. Total heat Evolved by Reaction (J) (4 + 5) ______________________

Trial 2

1. Volume of Salt Solution (mL) _________________________________2. Mass of Salt Solution (g) _____________________________________3. t of Salt Solution oC) (from graph) _____________________________4. Heat Gained by Salt Solution (J) ________________________________5. Heat Gained by Calorimeter (J) (Cal. Const. x 3) ___________________6. Total Heat Evolved by Reaction (J) (4 + 5) ______________________

Average Heat Evolved by Reaction (Q) (Trials 1&2) _______________________

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Name:_________________________________


Moles of Acid Used in the Neutralization Reaction

Moles Base of Used in the Neutralization Reaction

Moles of Water Formed in the Neutralization Reaction

Enthalpy of Neutralization (H) (Average Heat Evolved in kJ/ Moles Water

Formed)

Theoretical Enthalpy of Neutralization

From Standard Heat of Formation Data and calculate H in kJ per one mole of H2O.

Remember Ho

= nHfProducts nHfReactants

Physical Properties of 1.00 M Salt Solutions

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Solution Density, g/mL Specific Heat, J/g-oC

H2O 1.00 4.18

NaCl 1.04 3.89

KCl 1.04 3.10

NH4Cl 1.02 3.97

NaClO4 1.07 3.85NaNO3 1.06 3.81

KNO3 1.05 3.81

NH4NO3 1.03 3.93

Na2SO4 1.06 3.77

K2SO4 1.06 3.77

(NH4)2SO4 1.03 3.89

Na3PO4 1.11 3.81

K3PO4 1.12 3.85

(NH4)3PO4 1.09 3.89

Heat of Formation Data for 1.00 M Solutions at 25oC

Substance Hof, kJ/mol

H2O -285.8

HCl -167.4

HClO4 -129.3

HNO3 -207.5

H2SO4 -909.2

H3PO4 -1277.4

NaOH -469.4

KOH -479.9

NH4OH -362.3

NaCl -407.1

KCl -418.8

NH4Cl -299.6

NaClO4 -369.4

NaNO3 -446.4

KNO3 -459.8

NH4NO3 -339.7

Na2SO4 -1387.0

K2SO4 -1408.8

(NH4)2SO4 -1174.4Na3PO4 -1987.4

K3PO4 -2032.9

(NH4)3PO4 -1674.9

Name:_________________________________


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Prestudy Page 1

A student studied the enthalpy of neutralization of HClO4 and NaOH by reacting 50.0 mL

of 2.00 M HClO4 with 50.0 mL of 2.00 M NaOH in a calorimeter. He measured the

temperature versus time for 15 minutes. The first 4.5 minutes show the temperature of theacid and base before mixing. The solutions were mixed at the 5 minute mark and the

temperature of the resulting salt solution measured for 10 minutes to give the following

data.

Time, Minutes Solution Temperature,oC

0.0 (Acid/Base) 23.25

0.5 (Acid/Base) 23.27

1.0 (Acid/Base) 23.28

1.5 (Acid/Base) 23.30

2.0 (Acid/Base) 23.30

3.0 (Acid/Base) 23.354.0 (Acid/Base) 23.44

4.5 (Acid/Base) 23.47

5.0 Acid & Base Mixed

6.0 (Salt Solution) 33.50







13.0 (Salt Solution) 32.9614.0 (Salt Solution) 32.80


Plot a graph of temperature versus time and determine the t at the time of mixing, 5.0minutes. See Figure 2.

The density of the final solution is 1.07 g/mL and its specific heat is 3.85 J/g-oC.

The calorimeter constant is 85.8 J/oC.

Write a balanced equation for the reaction and calculate the moles of water formed.

Name:_________________________________


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Prestudy Page 2

Calculate the enthalpy (H) of neutralization in kJ per1.00 mole of water formed.RememberH = heated absorbed by the final salt solution plus the heat absorbed by the

calorimeter.

Calculate the theoretical (true) enthalpy of neutralization in kJ per 1.00 mole water from

heats of formation data given on page 12. H = nHfproducts nfreactants.

Calculate the % error between the two results.

% Error = (Exptl Value True Value) x 100% divided by True Value% Error can be + or depending whether the experimental value is greater or less than

the true value.

Enthalpy_of_Neutralization

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